The studies were performed to investigate the mechanism of phenamiphos affecting muscarinic acetylcholine receptor (M-AchR) of the brain synaptosome in rats. The results showed that phenamiphos inhibited the binding of 3H-quinuclidinyl benzilate (3H-QNB) to the brain synaptosome in rats, and this effect was dose and time-dependent. This suggests that phenamiphos might directly affect M-AchR. The KD and Bmax of phenamiphos to M-AchR were measured and found to be 1.3 x 10(-7) mol/L and 4.57 pmol/mg protein, respectively. It belongs to non-specific inhibition of phenamiphos on the binding of 3H-QNB to M-AchR. It was also found that phenamiphos decreased the level of protein sulfhydryl group in brain synaptosome membrane. Dithiothreitol (DTT) and the reduced glutathion (GSH) prevented the decrease of protein sulfhydryl group and the inhibition of phenamiphos on the binding of 3H-QNB to M-AchR. These showed that phenamiphos could affect the sulfhydryl group of M-AchR surface.